Tracing water mass fractions in the deep western Indian Ocean using fluorescent dissolved organic matter

The meridional distributions of fluorescent dissolved organic matter (FDOM) and various hydrologic properties were investigated along 67 degrees E in the western Indian Ocean. Our results showed that the highest fluorescence of the humic FDOM (FDOMH) was discovered in the Indian Deep Water (IDW), and relatively lower values were observed in the intruding water masses from the upper layer (e.g., Circumpolar Deep Water (CDW), Antarctic Intermediate Water (AAIW), and South Indian Central Water (SICW)). The deep FDOMH was robustly correlated with apparent oxygen utilisation (AOU), as suggested by previous studies. In particular, the slopes of the regression line AOU on FDOMH varied for different water masses and the two humic components. In this study, to identify the factor inducing the variations of the slope, we estimated the relative water mass fraction of different water masses using a three-end-member mixing model with a salinity-FDOMH diagram. The distribution of water mass fractions was in good agreement with water mass distribution from the conventional method from temperature and salinity distribution and previous studies. The FDOMH components were positively correlated with the aged water mass fraction (i.e., IDW; r = 0.93) and negatively correlated with fresher ones originating from the upper water (r = -0.93, -0.51, and -0.95 for CDW, AAIW, and SICW, respectively). The fluorescence ratio between the two FDOMH components was also observed to be linked to the water mass fractions. The results indicate that the distribution of FDOMH is attributed to the mixing of various deep-water masses during the global ocean circulation.